Heterogeneous-homogeneous reactions chain termination

Most examples of catalysis can be readily characterised as homogeneous or heterogeneous but there are examples of catalysis which overlap the two types. Consider a system in which intermediates are formed at the surface and then are desorbed into the gas phase and react there. Such intermediates might generate a chain reaction in the gas phase, i.e., chain initiation and chain termination occur at the surface but chain propagation occurs in the gas phase. 

Chain termination may be either heterogeneous or homogeneous in nature. At low pressure the chain carriers and branching agents may diffuse to the walls of the reactor where they are destroyed either by capture or conversion to inactive products. These heterogeneous termination reactions are either diffusion-controlled in which case the rate coefficient is given by the expression... 

Unfortunately, later the development of two areas— homogeneous kinetics and heterogeneous catalysis—occurred almost independently, which caused serious intrinsic discrepancies. For instance, the traditional chain theory implies the participation of surfaces also in chain termination, which determines the existence of the low-pressure ignition limit. In the framework of this approach, two regimes—diffusional and kinetic—are distinguished. In the latter case the parameter that describes the process is the probability of surface decay of chain carriers per one collision. It is worth noticing, however, that this assumes only a disappearance of active species from the gas phase, without any analysis of its mechanism and even stoichiometry. This is why the heterogeneous termination reactions are usually represented in kinetic models as a formal reaction ... 

Much effort has been devoted during the last 30 years toward understanding the mechanisms operative in the coordination catalysis of ethylene and a-olefin polymerization using Ziegler-Natta systems (metal halide and aluminum alkyl, sometimes with Lewis base modifiers). Aspects of the complex heterogeneous reactions have been elucidated (jL- ) but the intimate mechanistic detail - for example the role of inhibitors and promoters, kinetics and thermodynamics of chain growth, modes of chain transfer and termination - comes primarily from studies of homogeneous catalysts ... 

The closed sequence radical reaction introduces a multiplicative kinetic factor with respect to the initiation so that there is a certain analogy with homogeneous and heterogeneous catalysis, which is also characterized by the existence of closed sequences. However, a big difference exists between chain reactions and so-called catalytic reactions. In fact, in catalytic reactions, the total number of active centres is fixed, whereas that of chain reactions is determined by the competition between the initiation and termination reactions. The rate of a chain reaction is therefore a function of both the kinetic characteristics of the initiation, propagation and termination reactions. 

The interpretation of these results is based on the facts that, in the presence of oxygen, chain reactions of decomposition and of oxidation are competitive and that it appears new homogeneous processes of free-radical production (which result in an accelerating effect of O2) and new termination processes at least are partly heterogeneous Cwhich result in an inhibiting effect),... 

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